Printing method and printing apparatus

ABSTRACT

The temperature of a portion that belongs to a stencil and is located in the vicinity of a portion retaining a printing paste is increased to reduce the viscosity of printing paste that adheres to the portion retaining the printing paste, thereby allowing the printing paste to be easily separated from the retaining portion for the achievement of easy printing on an object on which a print is to be formed.

TECHNICAL FIELD

The present invention relates to a printing method for transferring aprinting paste retained on a stencil (plate) onto an object on which aprint is to be formed and printing apparatus for implementing theprinting method.

BACKGROUND ART

Conventionally, according to a planographic stencil (screen) typeprinting for printing, for example, solder paste on lands of a printedcircuit board, as shown in FIG. 18A and FIG. 18B, a screen mask (metalmask) 1 having through holes 1 a arranged in a specified pattern incorrespondence with lands 5 of the printed circuit board 4 is placed ina specified position on the board 4 while being brought in contact withthe board. Next, as shown in FIG. 18C, FIG. 19A and FIG. 19B, solderpaste 2 is supplied to one end of the screen mask 1, and this solderpaste 2 is moved by a squeegee 3 from the one end of the screen mask 1in a specified direction, thereby filling the solder paste 2 into eachthrough hole 1 a of the screen mask 1. Next, as shown in FIG. 18D, thescreen mask 1 is separated from the board 4 so as to move the solderpaste 2 inside the through holes 1 a of the screen mask 1 onto the lands5 of the board 4, thereby forming solder paste layers 2 a on the lands 5of the board 4 as shown in FIG. 18E.

However, according to the above structure, as shown in FIG. 19C, part ofthe solder paste 2 is left inside the through hole la of the screen mask1 while adhering to the inner wall of the through hole due to theviscosity of the solder paste itself when the screen mask 1 is separatedfrom the board 4, and this disadvantageously causes a phenomenon thatthe solder paste continuously extends across the left solder paste 2 andthe solder paste 2 placed on the land 5 of the board 4. Consequently, asthe screen mask 1 moves away from the board 4, the relative deformation(shear rate gradient) of the continuously extending solder pasteincreases to be pulled and broken at an arbitrary portion between thescreen mask 1 and the board 4. Part of the solder paste that has beenpulled and broken adheres to a portion other than the land 5 on theboard 4 as shown in FIG. 19D and adheres to the peripheral portion ofthe through hole 1 a on the rear surface of the screen mask 1 on theboard side. This has disadvantageously caused a printing blur in thenext printing stage, the occurrence of a bridge defined by theinadvertent adhesion of the solder paste to the adjacent solder pastelayer 2 a on the board 4, and the insufficient formation of a solderpaste layer on the board due to the adhesion of the solder paste to thescreen mask.

Accordingly, the object of the present invention is to solve theaforementioned issues and provide a printing method and printingapparatus capable of accurately pulling and breaking the printing pastebetween the stencil on which the printing paste is retained and theboard while causing no bridging, causing no printing blur attributed tothe printing paste left on the stencil side and causing no shortage ofsupply of the printing paste onto the board.

DISCLOSURE OF INVENTION

In order to achieve the above object, the present invention isconstructed so that the temperature of the portion which belongs to thestencil and on which the printing paste of the stencil is retained isincreased so as to reduce the viscosity of the printing paste thatadheres to the printing paste retaining portion and allow the printingpaste to be easily separated from the retaining portion, thereby makingthe printing paste easy to be printed on the object on which a print isto be formed.

According to a first aspect of the present invention, there is provideda printing method comprising:

retaining on a stencil a printing paste having a characteristic that aviscosity reduces as temperature increases;

increasing a temperature of a portion which belongs to the stencil andon which the printing paste is retained so as to reduce the viscosity ofthe printing paste to be brought in contact with the portion, therebymaking the printing paste easy to separate from the stencil; and

separating the printing paste retained on the stencil from the stencilso as to print the printing paste on an object on which a print is to beformed.

According to a second aspect of the present invention, there is provideda printing method based on the first aspect, wherein the portion whichbelongs to the stencil and on which the printing paste is retained isheated by electromagnetic induction heating to increase the temperatureof the portion.

According to a third aspect of the present invention, there is provideda printing method based on the second aspect, wherein the stencil has anopening portion to be arranged in a specified pattern for retaining theprinting paste, and the stencil and the object are relatively separatedapart after the stencil comes in contact with the object, therebyprinting the printing paste inside the opening portion onto the object.

According to a fourth aspect of the present invention, there is provideda printing method based on the third aspect, wherein an electromagneticinduction heating unit for performing the electromagnetic inductionheating performs the electromagnetic induction heating of the stencil ina noncontact manner.

According to a fifth aspect of the present invention, there is provideda printing method based on the fourth aspect, wherein an intervalbetween the electromagnetic induction heating unit and the stencil isconstructed so as to have a dimension such that a specified inductioncurrent flows through the stencil by the electromagnetic inductionheating unit.

According to a sixth aspect of the present invention, there is provideda printing method based on the third aspect, wherein an electromagneticinduction heating unit for performing the electromagnetic inductionheating performs the electromagnetic induction heating of the stencil ina contact manner.

According to a seventh aspect of the present invention, there isprovided a printing method based on any one of the third through sixthaspects, wherein the electromagnetic induction heating is performedafter retention of the printing paste on the opening portion of thestencil is finished.

According to an eighth aspect of the present invention, there isprovided a printing method based on any one of the third through seventhaspects, wherein the opening portion is a through hole, the stencil is ascreen mask, and the printing paste is filled into the through hole bymoving a squeegee.

According to a ninth aspect of the present invention, there is provideda printing method based on any one of the third through eighth aspects,wherein a print state is detected after the printing paste is printed onthe object, and an electromagnetic induction heating condition of thestencil or a condition of separation of the stencil from the object iscontrolled on the basis of a result of detection.

According to a tenth aspect of the present invention, there is provideda printing method based on any one of the third through ninth aspects,wherein the print material has a temperature gradient such that theportion put in contact with the portion retained by the stencil has ahigh temperature and the temperature gradually reduces departing fromthe portion in the electromagnetic induction heating.

According to an eleventh aspect of the present invention, there isprovided a printing method based on any one of the third through tenthaspects, wherein an induction current for generating the electromagneticinduction heat flows in the lengthwise direction of the opening portionof the stencil.

According to a twelfth aspect of the present invention, there isprovided a printing apparatus comprising:

a heating unit for increasing a temperature of a portion which belongsto a stencil for retaining a printing paste having a characteristic thata viscosity reduces as temperature increases and on which the printingpaste is retained so as to reduce the viscosity of the printing paste tobe brought in contact with the portion, thereby making the printingpaste easy to separate from the stencil; and

a printing paste separation unit for separating the printing pasteretained on the stencil from the stencil so as to print the printingpaste on an object on which a print is to be formed.

According to a thirteenth aspect of the present invention, there isprovided a printing apparatus based on the twelfth aspect, furthercomprising a stencil for retaining the printing paste having acharacteristic that its viscosity reduces as its temperature increases.

According to a fourteenth aspect of the present invention, there isprovided a printing apparatus based on the twelfth or thirteenth aspect,further comprising an electromagnetic induction heating unit for heatingby electromagnetic induction heating the portion which belongs to thestencil and on which the printing paste is retained, thereby increasingthe temperature of the portion.

According to a fifteenth aspect of the present invention, there isprovided a printing apparatus based on the twelfth or thirteenth aspect,wherein the stencil has an opening portion to be arranged in a specifiedpattern for retaining the printing paste, and the separation unitseparates the stencil relatively from the object after the stencil comesin contact with the object, thereby printing the printing paste insidethe opening portion onto the object.

According to a sixteenth aspect of the present invention, there isprovided a printing apparatus based on the fifteenth aspect, wherein theelectromagnetic induction heating unit for performing theelectromagnetic induction heating performs the electromagnetic inductionheating of the stencil in a noncontact manner.

According to a seventeenth aspect of the present invention, there isprovided a printing apparatus based on the sixteenth aspect, wherein aninterval between the electromagnetic induction heating unit and thestencil is constructed to have a dimension such that a specifiedinduction current flows through the stencil by the electromagneticinduction heating unit.

According to an eighteenth aspect of the present invention, there isprovided a printing apparatus based on the fifteenth aspect, wherein theelectromagnetic induction heating unit for performing theelectromagnetic induction heating performs the electromagnetic inductionheating of the stencil in a contact manner.

According to a nineteenth aspect of the present invention, there isprovided a printing apparatus based on any one of the fifteenth througheighteenth aspects, wherein the electromagnetic induction heating isperformed after retention of the printing paste on the opening portionof the stencil is finished.

According to a twentieth aspect of the present invention, there isprovided a printing apparatus based on any one of the fifteenth throughnineteenth aspects, wherein the opening portion is a through hole, thestencil is a screen mask, and the printing paste is filled into thethrough hole by moving a squeegee.

According to a twenty-first aspect of the present invention, there isprovided a printing apparatus based on any one of the fifteenth throughtwentieth aspects, further comprising a control section for detecting aprint state after the printing paste is printed on the object andcontrols an electromagnetic induction heating condition of the stencilor a condition of separation of the stencil from the object on the basisof a result of detection.

According to a twenty-second aspect of the present invention, there isprovided a printing apparatus based on any one of the fifteenth throughtwenty-first aspects, wherein the print material has a temperaturegradient such that the portion put in contact with the portion retainedby the stencil has a high temperature and the temperature graduallyreduces departing from the portion in the electromagnetic inductionheating.

According to a twenty-third aspect of the present invention, there isprovided a printing apparatus based on any one of the fifteenth throughtwenty-second aspects, wherein an induction current for generating theelectromagnetic induction heat flows in the lengthwise direction of theopening portion of the stencil.

According to the above aspects of the present invention, the stencilitself is heated by induction heating, so that the temperature of theprinting paste portion retained by the stencil (the portion of theprinting paste that comes into contact with the inner wall surface ofthe through hole of the stencil and the portion in the vicinity of theportion) is increased more than in the inner portion, resulting in areduced viscosity. As a result, the adhesive force of the printing pastebetween the stencil and the printing paste is reduced, as a consequenceof which a resistance force when the printing paste is easily separatedfrom the stencil becomes small to allow the stencil separation operationto be satisfactorily achieved. Therefore, no printing paste is left onthe stencil side, so that no blur of printing is caused in the nextprinting stage and a specified amount of printing paste is supplied,that is, the printing paste is supplied in a specified shape to aspecified position on the object on which a print is to be formed,thereby allowing a printing paste layer to be formed by printing.According to the above aspects of the present invention, the resistanceof the printing paste in the inner wall surface portion of the throughhole of the stencil becomes small. Therefore, a satisfactory printresult can be obtained even when the stencil separation velocity is sethigher (for example, not smaller than 1 mm/s and not greater than 3mm/s) than the conventional stencil separation velocity (for example,not smaller than 0.1 mm/s and smaller than 1 mm/s) or without velocitycontrol.

According to the above induction heating, the stencil itself generatesheat, and therefore, the discharge of heat of the stencil can beimmediately performed after the stop of the induction heating.Therefore, the portion other than the stencil is not heated, exerting nobad influence on the next printing operation, the devices around thestencil and so on. In contrast to this, according to the method ofheating the stencil by externally radiating heat as observed in the caseof hot air, radiation heating (infrared heating), or conduction heating,the members and air around the stencil are heated and the members andair around the heating unit, which also generates heat, aredisadvantageously heated. Therefore, bad influence is sometimes exertedon the next printing operation, the devices around the stencil and soon. According to the method of transmitting heat from the heating unitto the stencil, the heat is conducted not only to the stencil but alsoto the heating unit and the members and air around the stencil,resulting in the drawback that heating efficiency is bad.

When performing induction heating in a noncontact manner withoutbringing the induction heating unit in contact with the stencil, theinduction heating unit is not brought in contact with the printing pasteon the surface of the stencil, and therefore, the induction heating unitis not smeared by the printing paste. In the case where an electroniccomponent exists on the lower surface of the object on which a print isto be formed, the noncontact method can prevent the exertion of badinfluence on the electronic component during the induction heatingbecause of an increased distance from the electronic component.

BRIEF DESCRIPTION OF DRAWINGS

These and other aspects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiments thereof with reference to the accompanyingdrawings, in which:

FIG. 1A, FIG. 1B, FIG. 1C, and FIG. 1D are, respectively, explanatoryviews for explaining a printing method according to one embodiment ofthe present invention;

FIG. 2 is a block diagram of a printing apparatus according to oneembodiment of the present invention;

FIG. 3 is a perspective view of the printing apparatus of FIG. 2;

FIG. 4 is a flow chart of the printing operation of the printingapparatus of FIG. 2;

FIG. 5 is a sectional view of a screen mask in a state in which thescreen mask is heated by a screen mask heating unit of the aboveprinting apparatus;

FIG. 6 is a perspective view of an induction coil of the above screenmask heating unit of FIG. 5;

FIG. 7A, FIG. 7B, and FIG. 7C are a graph of a viscosity distribution ofsolder paste, a graph of a temperature distribution, and a state of thesolder paste inside the through hole of the screen mask, respectively;

FIG. 8 is a graph showing a relation between the temperature and theviscosity of the solder paste;

FIG. 9 is a sectional view of one embodiment of the present invention,in which the screen mask heating unit is in direct contact with thescreen mask;

FIG. 10A, FIG. 10B, FIG. 10C, and FIG. 10D are explanatory views ofstates in which through holes of the screen mask are arranged in theX-direction, Y-direction, and at an angle of 45 degrees, and aperspective view of a QFP having a pattern of through holes as shown inFIG. 10C, respectively;

FIG. 11A and FIG. 11B are a graph showing a relation between a distancefrom the inner wall of the through hole of the screen mask and ashearing force and an explanatory view thereof, respectively;

FIG. 12A and FIG. 12B are a perspective view of a filling roller in oneembodiment of the present invention that employs the cylindrical fillingroller in place of a squeegee and a partially sectional explanatory viewof a print state achieved by the filling roller, respectively;

FIG. 13A and FIG. 13B are an explanatory view of one embodiment of thepresent invention that takes advantage of an extruding function of apiston in place of a squeegee and an explanatory view of one embodimentof the present invention that takes advantage of an extruding functionby compressed air, respectively;

FIG. 14 is an explanatory view of one embodiment of the presentinvention in the case where the present invention is applied to adirect-printing planographic transfer printing system;

FIG. 15A and FIG. 15B are explanatory views of one embodiment of thepresent invention in the case where the present invention is applied toan offset printing system, respectively;

FIG. 16 is an explanatory view of one embodiment of the presentinvention in the case where the present invention is applied to aplanographic intaglio transfer printing system;

FIG. 17 is an explanatory view of one embodiment of the presentinvention in the case where the present invention is applied to anintaglio transfer printing system (gravure printing system);

FIG. 18A, FIG. 18B, FIG. 18C, FIG. 18D, and FIG. 18E are, respectively,explanatory views showing a prior art screen printing system;

FIG. 19A, FIG. 19B, FIG. 19C, and FIG. 19D are, respectively,explanatory views showing a prior art screen printing system;

FIG. 20 is a perspective view of an X-direction driving unit accordingto the above embodiment of the present invention;

FIG. 21 is a perspective view of a stencil separation unit (Z-directiondriving unit) according to the above embodiment of the presentinvention;

FIG. 22 is a perspective view of another stencil separation unit(Z-direction driving unit) according to the above embodiment of thepresent invention;

FIG. 23 is a perspective view of a rectangular induction coil accordingto another embodiment of the present invention;

FIG. 24 is a perspective view showing a state in which two inductioncoils of FIG. 23 are prepared and arranged at two corners located indiagonal positions of a QFP so as to flow an induction current in thelengthwise direction of each through hole;

FIG. 25 is a perspective view showing a state in which four inductioncoils of FIG. 23 are prepared and arranged at the four corners of a QFPso as to flow an induction current in the lengthwise direction of eachthrough hole; and

FIG. 26 is a perspective view showing a state in which one inductioncoil of FIG. 23 is prepared and arranged above a QFP in a form inclinedat an angle of 45 degrees with respect to the through hole pattern so asto flow an equal amount of induction current through each through hole.

DETAILED DESCRIPTION

Before the description of the present invention proceeds, it is to benoted that like parts are designated by like reference numeralsthroughout the accompanying drawings.

Embodiments according to the present invention will be described indetail below with reference to FIG. 1A through FIG. 17 and FIGS. 20, 21,and 22.

A printing method according to one embodiment of the present inventionis related to a planographic stencil (screen) type printing method forprinting a printing paste such as solder paste on lands of a printedcircuit board as shown in FIG. 1A through FIG. 1D. The printing methodaccording to this embodiment is as follows. First, as shown in FIG. 1A,a screen mask (metal mask) 11 having through holes 11 a arranged in aspecified pattern in correspondence with the lands 15 of a printedcircuit board 14 is placed in a specified position on the board 14 whilebeing brought in contact with the board. Next, a solder paste 12 issupplied onto one end of the screen mask 11, and this solder paste 12 ismoved by a squeegee 13 in a specified direction from the one end of thescreen mask 11, thereby filling the solder paste 12 into the throughholes 11 a of the screen mask 11. Next, as shown in FIG. 1B, thetemperatures of the inner wall surfaces of the through holes 11 a of thescreen mask 11 are increased by induction heating. In this stage, thetemperatures of the inner wall surfaces are increased to temperatures atwhich the viscosity of the solder paste 12 to be used is reduced tobecome hard to adhere to the inner wall surfaces of the through holes11a of the screen mask 11. Next, as shown in FIG. 1C, the screen mask 11is separated from the board 14, so that the solder paste 12 inside thethrough holes 11a of the screen mask 11 is moved onto the lands 15 ofthe board 14, thereby forming solder paste layers 12 a on the lands 15of the board 14 as shown in FIG. 1D. In this stage, the viscosity of thesolder paste 12 is reduced by the induction heating, and therefore, thesolder paste 12 inside the through holes 11a of the screen mask 11scarcely adheres to the inner wall surfaces of the through holes 11 a.Therefore, the solder paste 12 inside the through holes 11 a is leftformed as the solder paste 12 is on the lands 15 of the board 14 evenwhen the screen mask 11 is separated from the board 14, so that thesolder paste layers 12 a of specified shapes can be formed in specifiedpositions.

The printing method according to the above embodiment can be implementedby a printing apparatus according to one embodiment of the presentinvention as shown in FIGS. 2 and 3. More concrete operation of theprinting method executed by this printing apparatus is shown in theflowchart of FIG. 4.

In the printing apparatus shown in FIG. 2, a board carrying-in andcarrying-out unit 21 provided with a carrying-in unit 21 a and acarrying-out unit 21 b, a board support unit 22, a screen mask 11, asqueegee head driving unit 24, a stage section 20 provided with anXY_(θ)-position correcting unit 25 and a stencil separation unit 26, anda screen mask heating unit 27 provided with an induction heating section28 and a timer 29 can be respectively driven under control of a controlsection 34. The control section 34 receives the inputs of board positionrecognition correction information from a board position recognizing andcorrecting section 30 provided with a processing operation section 31and a recognition camera section 32 as well as print inspectioninformation from a print inspecting section 38 provided with aprocessing operation section 39, a print state detecting means 40, andan inspection criteria storage section 41. The control section 34 inputsprocess information to and outputs process information from a processcontrol section 35 provided with a processing operation section 36 andan acceptable product print database 37 and receives the input ofinformation of the print state from the print inspecting section 38,thereby executing process control. The control section 34 displays theresults of operation and inspection, the state of the printed solderpaste 12, and so forth on a display section 33 as the occasion demands.

The board 14 is carried in to the stage section 20 by the carrying-inunit 21 a of the board carrying-in and carrying-out unit 21, correctedin position in the stage section 20, thereafter moved to a printingposition, printed in the printing position, and thereafter carried outof the printing apparatus from the printing position by the carrying-outunit 21 b of the board carrying-in and carrying-out unit 21.

In the stage section 20, first, the board 14 is retained in position bythe board support unit 22 provided in the stage section 20. The board 14is retained in position by, for example, a method for vacuum-sucking theboard 14 with a number of suction holes opened on the surface of theboard support unit 22, a method for supporting the lower surface of theboard 14 by means of a number of backup pins or the like. In the statein which the board 14 is retained in position, a position correctingmark(s) (not shown) of the board 14 is recognized by the recognitioncamera section 32 of the board position recognizing and correctingsection 30. The processing operation section 31 calculates a positionaldisplacement between the recognized position of the board 14 and theposition of the screen mask 11, and calculates the position correctionamount of the board 14 for correcting this positional displacement. Thisresult of calculation is inputted to the XY_(θ)-position correcting unit25 of the stage section 20. On the basis of this inputted positioncorrection information, the positional correction of the board 14relative to the screen mask 11 is executed by the XY_(θ)-positioncorrecting unit 25 of the stage section 20. That is, the XYθ-positioncorrecting unit 25 executes the positional correction of the board 14 inthe orthogonal XY-directions along the horizontal plane of the printingapparatus and in a θ-direction around the Z-axis in the verticaldirection relative to the screen mask 11 on the basis of the aboveposition correction information. The XYθ-position correcting unit 25 isconstructed so that a Y-direction table 25 b capable of moving in theY-direction is placed on an X-direction table 25 a capable of moving inthe X-direction (direction in which the board is carried in and out) anda θ-direction table 25 c capable of turning in the θ-direction isfurther placed on them. Then, by moving each of the tables in therespective directions by the position correction amount, the positionalcorrection of the board 14 is executed. It is to be noted that thepositional correction in the X-direction is executed by the X-directiontable 25 a after the positional correction in the Y-direction and theθ-direction is completed and before the board 14 is brought in contactwith the screen mask 11 after the board 14 is moved to the printingposition and stopped there.

An X-direction driving unit 20 x that concurrently serves as thisX-direction position correcting unit is shown in FIG. 20. In FIG. 20,the X-direction table 25 a is arranged movably in the X-direction alonga pair of linear guides 25 m extending in the X-direction, and athreaded shaft 25 n is rotated forwardly and reversely by driving adriving motor 25 p forwardly and reversely, thereby moving forwardly orbackwardly the X-direction table 25 a fixed to a nut 25 r meshed withthe threaded shaft 25 n in the X-direction.

The board 14 retained by the board support unit 22 is moved in theX-direction to the printing position by the X-direction driving unit 20x of the stage section 20. In the printing position, the board 14 ispositioned below the screen mask 11 and moved up until the upper surfaceof the board 14 is brought in contact with the lower surface of thescreen mask 11 by the stencil separation unit 26. Then, in the state inwhich the lower surface of the screen mask 11 is put in contact with theupper surface of the board 14, the solder paste 12 is supplied to theone end in the X-direction of the screen mask 11, and the squeegee 13 ismoved by the squeegee head driving unit 24 from the one end to the otherend in the X-direction of the screen mask 11, thereby filling the solderpaste 12 into the through hole 11 a of the screen mask 11.

The screen mask 11 is constructed by forming, opening portions comprisedof through holes 11 a corresponding to copper-made conductor patternportions (lands) 15 of the board 14 through, for example, a plate madeof nickel or stainless steel having a thickness of about 150 μm.

The squeegee head driving unit 24 is to move the squeegee 13 on thescreen mask 11 in order to fill the solder paste 12 into the throughholes 11a of the screen mask 11. The squeegee 13 is constructed of aflat plate or a plate having a sword-like (roughly pentagonal)cross-section shape. The squeegee 13 is moved on the screen mask 11 byforwardly and reversely rotating a ball thread 24 b by the driving of amotor 24 c and forwardly and backwardly moving a squeegee head 24 ameshed with the ball thread 24 b in the axial direction of the ballthread 24 b. The squeegee head 24 a can be moved up and down by theforward and reverse rotation of a motor 24 d. The tilt angle of thesqueegee 13 itself relative to the screen mask 11 can also be adjustedby a cylinder 24 f. That is, the squeegee 13 is pivotally supported at aportion that is not shown, and by upward or downward moving one end ofthe squeegee 13 by driving the cylinder 24 f, the tilt of the squeegee13 can be adjusted by pivoting the squeegee 13 around the above supportpoint used as a fulcrum.

The solder paste 12 filled in each of the through holes 11 a of thescreen mask 11 comes to have a lower end surface put in contact witheach of the lands 15 of the board 14 corresponding to the through holes11 a, and by separating the screen mask 11 from the board 14 by thestencil separation unit 26, the solder paste layers 12 a are formed onthe lands 15 of the board 14.

An example of the stencil separation unit 26 is shown in FIG. 21. InFIG. 21, a driving nut 25 v is rotated forwardly and reversely via abelt 25 u by the forward and reverse rotational driving of an ACservomotor 25 t so as to upwardly or downwardly move a threaded shaft 25w 1 h mesh with the nut 25 v, thereby upwardly or downwardly moving theboard support unit 22 fixed to the upper end of the threaded shaft 25 wfor the upward or downward movement of the board 14. Therefore, when theboard 14 is moved in the X-direction from a board position correctingoperation position to the printing position below the screen mask 11 bythe X-direction driving unit 20 x of the stage section 20, the ACservomotor 25 t of the stencil separation unit 26 is driven to move upthe board 14 until the upper surface of the board 14 is brought incontact with the lower surface of the screen mask 11. After thecompletion of the printing, the board 14 is moved down relative to thescreen mask 11 by the driving of the AC servomotor 25 t of the stencilseparation unit 26 in order to effect the stencil separation operation.The board 14, separated from the stencil, is carried out of the printingapparatus by the carrying-out unit 21 b.

FIG. 22 shows another stencil separation unit. FIG. 22 shows a stagesection (board support unit) 402, an AC servo controller 417, an ACservomotor 414 to be controlled by the AC servo controller 417, a ballthread 408 to be rotated forwardly and reversely by an AC servomotor414, an upper bearing 409 of the ball thread 408, a lower bearing 410 ofthe ball thread 408, a pulley 411 on the ball thread 408 side, a pulley412 on the AC servomotor 414 side, a timing belt 413, and a linear guide415 that guides the upward and downward movement of the stage section402. This stencil separation unit is constructed so that the stagesection (board support unit) 402 can move up and down at an arbitrarilyset velocity within an arbitrarily set range by the AC servo controller417, the AC servomotor 414 and the ball thread 408, whereby the stencilseparation velocity of the board 14 with respect to the screen mask 11can be arbitrarily adjusted.

Immediately before executing the stencil separation operation, i.e.,immediately after the completion of the printing with the solder paste12, the screen mask 11 is heated by induction heating by the screen maskheating unit 27. In the screen mask heating unit 27, as shown in FIGS. 5and 6, a ring-shaped induction coil 28 a of the induction heatingsection 28 is arranged in a state in which the coil is separated apartby a specified distance above the screen mask 11. When the solder paste12 is filled into the through holes 11 a of the screen mask 11, anelectric current is made to flow through the induction coil 28 a for atime set by the timer 29, that is, for example, a time within severalmilliseconds to several seconds so as to generate an induction magneticfield and flow an induction current through the screen mask 11 itself,thereby directly heating the screen mask 11 itself by the inductionheating. An example of this induction coil 28 a has a circulardoughnut-shape with an inner wire diameter of 50 mm, an outer diameterof 170 mm and a thickness of 2 mm, and the induction coil is constructedby winding 35 conductive enameled wires or copper wires having a lowelectrical resistance (generating no Joule's heat) by the number ofturns h=21. In regard to induction heating conditions, the inductionheating is executed by supplying an electric power of 1400 W for severalseconds at 100 V and 60 Hz. In the present embodiment, the inductioncoil 28 a is arranged a specified interval apart from the upper surfaceof the screen mask 11 in a noncontact manner as shown in FIG. 5. It isacceptable to retreat the coil from above the screen mask 11 during theprinting of the solder paste 12 so as not to hinder the printing of thesolder paste 12 and move the coil to the place above the screen mask 11during the induction heating so as to allow the induction heating to beeffected. It is preferred that the interval between the induction coil28 a and the screen mask 11 is arranged to a dimension such that aspecified current flows through the screen mask 11 by the induction coil28 a.

During the induction heating, an induction current flows since thescreen mask 11 is made of a conductive material such as stainless steel.However, the stainless steel or the like has a greater resistance thancopper, and therefore, the screen mask itself generates heat. Incontrast to this, the solder paste 12, which has a small solder particlediameter or a cream-like form due to the flux, exhibits no electricconductivity, so that no induction current flows and no heat isgenerated. Therefore, as shown in FIG. 7, if the screen mask 11 isheated by the induction heating, then the temperatures of the inner wallsurfaces of the through holes 11 a of the screen mask 11 are increased.Therefore, the temperature increases in the portion which belongs to thesolder paste 12 and is put in contact with the inner wall surface ofeach through hole 11 a and around the portion. In contrast to this, thetemperature does not increase in the center portion of the solder paste12, so that a temperature gradient as shown in FIG. 7B is formed betweenthe center portion of the solder paste 12 and the peripheral portion(the portion in contact with the through hole 11 a). That is, the solderpaste 12 comes to have a temperature gradient such that the portion putin contact with the inner wall surface of the through hole 11 a is at ahigh temperature and the temperature is gradually reduced from theportion toward the center portion of the solder paste 12. Consequently,as shown in FIG. 7A, the viscosity of the solder paste 12 is reduced atthe peripheral portion than in the center portion. This is because thesolder paste 12 has a characteristic as shown in FIG. 8, i.e., thecharacteristic that the viscosity is reduced as the temperatureincreases. By this induction heating, the viscosity of the solder paste12 becomes reduced between the inner wall surface of the through hole 11a of the screen mask 11 and the solder paste 12 that is put in contactwith the inner wall surface, and thus the solder paste 12 is easilyseparated from the through hole 11a of the screen mask 11, meaning thatthe stencil separation is satisfactorily achieved.

One example of the material of the solder paste 12 should preferablyinclude 90 percent by weight of metal powder and 10 percent by weight offlux. The metal powder contains about 62 percent by weight of tin andthe other component of lead, and its particle diameter is 20 to 40 μm.The flux has a solvent of 75 to 40 percent by weight of alcohol and thelike and 25 to 60 percent by weight of other solid components. The solidcomponents include rosin, activator, and thixotropic agent. As aconcrete product name of the solder paste, there can be enumerated thesolder paste of a product number MR7125 having 63 percent by weight oftin and 37 percent by weight of lead, produced by Panasonic.

As a material of the screen mask 11, there can be enumerated astainless-steel-based metal (e.g., SUS304) of nickel-chrome system etc.,a nickel-based metal or the like. It is also acceptable to use a screenmask constructed by forming a conductive vapor-deposited film or aplating film on the surface of a synthetic resin such as polyimide andthe inner wall surface of the through hole. In this case, anelectromagnetic induction can be generated in the portion of theconductive vapor-deposited film or the plating film on the inner wallsurface of the through hole.

Furthermore, if the board 14 that serves as the object on which a printis to be formed is constructed of copper having an excellentconductivity, then no heat is generated in the board 14 by theelectromagnetic induction, causing no bad influence on the electroniccomponents and so forth on the board.

The print inspecting section 38 measures the state in which the solderpaste layer 12 a is formed on the land 15 of the board 14, i.e., theshape and position of the solder paste layer 12 a by means of a cameraor a laser length measuring instrument as an example of the print statedetecting means 40, and the volume and the amount of positionaldisplacement of the solder paste layer 12 a are calculated by theprocessing operation section 39 on the basis of the results ofmeasurement. The laser length measuring instrument applies a laser beamto the solder paste layer 12 a and calculates the height and so on ofthe solder paste layer 12 a from the position of the reflected light.The above results of calculation are compared with the inspectioncriteria stored in the inspection criteria storage section 41, and it isthen determined whether the print is good or not. The result of decisionis output to the control section 34, and if the print is defective, thecontents of the defect are numerically expressed and the numeric valueis also output to the control section 34. This deciding operation isexecuted by, for example, calculating the height, width, volume and soon of the solder paste layer 12 a from, for example, an image capturedby the camera of the print state detecting means 40 or position datameasured by the laser length measuring instrument, comparing thedecision data of the height, width, volume and so on of the solder pastelayer stored in the inspection criteria storage section 41 with theabove calculated values in the processing operation section 39, anddeciding whether the print is good or not.

The process control section 35 changes the parameter setting of theprinting apparatus on the basis of the post-printing data of the statein which the solder paste 12 is printed, made by the print inspectingsection 38. In this case, the above parameters include, for example, theparameters of each unit stored in the acceptable product print database37 (for example, printing velocity, tilt angle of the squeegee 13,environmental temperature during printing (for example, squeegeetemperature, screen mask temperature, board temperature, and temperatureof air and the like around them, enumerated in the order of importance),printing pressure, i.e., pressure of the squeegee 13 pressed against thescreen mask 11, stencil separation velocity of the board 14, and profileof acceleration) and induction heating conditions (for example, heatingoutput, heating time, and heating start timing). Relations between theparameters and the print quality are stored as database, and the optimumparameters are calculated by the processing operation section 36 of theprocess control section 35.

The above printing method to be implemented by the above printingapparatus will be described with reference to the flowchart of FIG. 4.It is to be noted that this sequence of operations is controlled by thecontrol section 34.

In step S1, the board 14 is carried in to the stage section 20 by thecarrying-in unit 21 a of the board carrying-in and carrying-out unit 21.

Next, in step S2, the board 14 carried in to the stage section 20 issupported by the board support unit 22.

Next, in step S3, the board position recognizing and correcting section30 recognizes the position of the board 14 retained by the board supportunit 22 and calculates the position correction amount of the board 14relative to the screen mask 11.

Next, in step S4, the positions in the XYθ-directions of the board 14relative to the screen mask 11 are each corrected by the XYθ-positioncorrecting unit 25 of the stage section 20 on the basis of the abovecalculated position correction amount.

Next, in step S5, the board 14 is positioned in the printing positionbelow the screen mask 11 by the stage section 20, and the board 14 ismoved up by the stage section 20 so that the screen mask 11 comes incontact with the upper surface of the board 14.

Next, in step S6, the squeegee 13 is moved on the screen mask 11,thereby filling the solder paste 12 into the through holes 11 a of thescreen mask 11.

Next, in step S7, it is determined whether or not the screen mask 11 isto be inductively heated. When the induction heating is not executed asin the case where the solder paste 12 easily separates from the throughholes 11 a, the program flow proceeds to step S8. When the inductionheating is executed, the program flow proceeds to step S9 to set thetimer 29 to a predetermined heating time. Immediately after thecompletion of the printing of the solder paste 12 in step S10, thescreen mask 11 is inductively heated by the induction coil 28 a of theinduction heating section 28.

When the induction heating is executed, the stencil separation operationis executed in step S8 immediately after the execution of the inductionheating. That is, by driving the stencil separation unit 26 of the stagesection 20, the board 14 is moved down relative to the screen mask 11 toseparate the board 14 from the screen mask 11 and transfer the solderpaste 12 from inside the through holes 11 a of the screen mask 11 ontothe lands 15 of the board 14. When no induction heating is executed, theabove stencil separation operation is executed after the completion ofthe printing of the solder paste 12, thereby transferring the solderpaste 12 from inside the through holes 11 a of the screen mask 11 ontothe lands 15 of the board 14.

Next, in step S12, the shapes, positions and so on of the solder pastelayers 12 a formed on the board 14 are inspected by the print inspectingsection 38.

Next, in step S13, it is determined whether the print state is good ornot based on the result of the above inspection. When it is determinedthat the print state is good, the program flow proceeds to step S14 tocarry the board 14 out of the printing apparatus by the carrying-outunit 21 b in step S14, and the sequence of printing operations ends.When it is determined that the print state is defective in step S13, theprogram flow proceeds to step S15 to change the design of the processparameter(s) by the process control section 35 and end the sequence ofprinting operations. The next printing of the solder paste 12 isexecuted on the basis of the optimum condition information obtainedthrough the design change in step S15, and the post-printing stencilseparation process in step S8 and the induction heating process of thescreen mask 11 in step S10 are executed. Depending on the particularcases, it is also acceptable to remove the solder paste layer(s) thathas been determined to be defectively printed, execute a new printingoperation under the condition(s) obtained through the design change instep S15 and execute the post-printing stencil separation process instep S8 and the induction heating process of the screen mask 11 in stepS10.

In this flowchart, the process of executing the stencil separationprocess in step S8 by changing the design of the stencil separationconditions and the induction heating process in steps S9 and S10 bychanging the design of the induction heating condition(s) are shown asan example. For this parameter design change, all the parameters are notsimultaneously changed in design, but only the appropriately selectedparameter(s) are changed in design according to the print state.

According to the above embodiment, the screen mask 11 itself is heatedby the induction heating, so that the outer peripheral portion of thesolder paste 12 put in contact with the inner wall surface of each ofthe through holes 11 a of the screen mask 11 comes to have a temperaturethat has increased more than that in the center portion, consequentlyacquiring a reduced viscosity. As a result, the adhesive force betweenthe inner wall surface of the through hole 11 a of the screen mask 11and the solder paste 12 is reduced, so that the solder paste 12 easilyseparates from the screen mask 11, thereby allowing the stencilseparation operation to be satisfactorily achieved. Therefore, thesolder paste 12 is not left on the screen mask 11 side causing no blurof print in the next printing stage, so that a specified amount ofsolder paste 12 can be supplied, that is, the solder paste 12 can besupplied in the specified shape to the specified position, therebyallowing the solder paste layer(s) 12 a to be formed as a print.

Furthermore, according to the above induction heating operation, thescreen mask 11 itself generates heat, so that the heat discharge of thescreen mask 11 can be performed immediately after the stop of theinduction heating operation. In addition, the members other than thescreen mask 11 are not heated, so that no adverse influence is exertedon the next printing operation, the peripheral units around the screenmask 11 and so on. In contrast to this, according to the method ofmerely radiating heat from the outside of the screen mask 11 to heat thescreen mask 11 as observed in the case of hot air, radiation heating(infrared heating), or conduction heating, the members and air aroundthe screen mask 11 are heated, and the members and air around theheating unit are heated due to the heating of the heating unit itself.This sometimes might adversly affect the next printing operation, theunits around the screen mask 11 and so on. Furthermore, according to themethod of conducting heat from the heating unit to the screen mask 11,heat is conducted not only to the screen mask 11 but also to the heatingunit and the members and air around the screen mask 11, thereby causinga drawback in that the heating efficiency is bad.

When executing the induction heating operation in a noncontact mannerwithout putting the induction heating section 28 in contact with thescreen mask 11, the induction coil 28 a of the induction heating section28 does not come in contact with the solder paste 12 left on the surfaceof the screen mask 11, and therefore, the induction coil 28 a is notsmeared by the solder paste 12. According to this noncontact method,when there is an electronic component(s) on the lower surface of theboard 14, the distance to the electronic component is increased, so thatthe possible exertion of negative influence on the electronic componentduring the induction heating can be prevented.

In this case, an experiment was conducted to determine the degree towhich the solder paste 12 of a fine pattern could be satisfactorilyseparated from the through holes 11 a by induction heating. The diameterof the through hole was about 0.1 mm, and the distance between thecenters of through holes, i.e., the pitch between adjacent through holeswas 0.2 mm. The environmental temperature of the air, solder paste,screen mask and so on around the through hole was 23° C. The experimentresults are shown in FIG. 11A and FIG. 11B. As shown in FIG. 11A andFIG. 11B, according to this experiment, the shearing force of the solderpaste 12 filled in the through hole 11 a of the screen mask 11 at thetime of separation from the stencil exhibits no reduction in shearingforce in the portion of a through hole pitch of not greater than 0.2 mm.This implied that a pitch of 0.2 mm is the limit of the fine printingand no significant reduction in shearing force could be expected when adistance d from the inner wall surface of the through hole was notgreater than 0.05 mm.

Therefore, according to the present invention, by taking advantage ofthe induction heating, the fine printing at a pitch of 0.3 mm, which hasbeen difficult, conventionally can be satisfactorily performed, and fineprinting can also be performed to the extent of a pitch of about 0.2 mmdepending on the conditions of the solder paste and so on.

With regard to the induction heating conditions, by supplying theelectric power of 1400 W for one to two seconds, the temperature of thescreen mask 11 can be increased to about 50 to 70° C. with interpositionof a gap of 1 mm. Furthermore, by setting the supply power to about 2000W, the equivalent temperature increase can be achieved within onesecond, whereby the solder paste inside the through hole 11 a is allowedto achieve a greater temperature difference between the inner wallsurface of the through hole 11 a and the center portion thereof. Byputting the induction coil in contact with the screen mask 11, a moreefficient temperature increase can be achieved.

According to the above induction heating stencil separation process, thetemperature difference between the inner wall surface of the throughhole 11 a and the center portion of the through hole 11 a depends on thethrough hole width. Therefore, according to the screen mask having aplurality of types of through hole widths, it is preferable that thecondition setting is performed in accordance with the minimum throughhole size among the plurality of through holes to be subjected to theinduction heating. That is, if the minimum through hole width is about0.15 mm, then, as described above, there is necessitated such a sharpcontrol as the supply of a power of 2000 W to the induction coil for asupply time of about one second. However, in the case of a plurality ofthrough holes which are so relatively roughly arranged so that theminimum through hole width of the through holes is 1 mm, then the supplypower is allowed to be 1000 W and the supply time is allowed to be twoto three seconds. Therefore, the heating conditions of the inductioncoil can be preparatorily determined according to the pattern of thescreen mask (i.e., the arrangement, size and so on of the throughholes). In such a case, the induction coil heating conditions conformingto the through hole size may be set to the heating conditions close tothe optimum characteristic values appropriate for the separation fromthe stencil of each through hole obtained from the previously-measuredcharacteristic values (viscosity, shearing stress value and yield valuewith respect to temperature) of the solder paste.

It is to be noted that the temperature control can also be executed byinduction heating when executing the mask cleaning of the screen mask11. By this operation, the solder paste left inside the through hole andthe rear surface of the screen mask can be more efficiently removed. Theconditions in this case are not required to be controlled strictly ascompared with the time of separation from the stencil, and the screenmask is required to be heated to such an extent that the solder pastecomes to have a good flowability. The heating may be executed with, forexample, a power of 1000 W during the cleaning time.

It is to be noted that the present invention is not limited to the aboveembodiment, and the invention can be implemented in a variety of forms.

For example, in the above embodiment, the board 14 is moved down in thestate in which the screen mask 11 is made stationary in order toseparate the screen mask 11 from the board 14 relative to each other.However, the present invention is not limited to this, and the screenmask 11 may be moved in a state in which the board 14 is stationary. Itis also acceptable to move both the screen mask 11 and the board 14 indirections in which they are separated from each other.

The printing paste is not limited to the solder paste 12, and anyarbitrary material may be used so long as the present invention can beapplied. For example, the material may be constructed of a metal powderhaving a minute particle diameter of not greater than about 200 μm and aflux instead of the solder paste. Examples, of this metal powder, assilver or copper.

In the case of screen printing, the induction heating is effected afterthe completion of the printing paste scraping operation by the squeegee.However, the present invention is not limited to this, and it isacceptable to start the induction heating simultaneously with thescraping operation, execute the heating at a temperature lower than thespecified heating temperature in the initial stage and increase thetemperature of the outer peripheral portion of the printing paste to theabove specified temperature by the induction heating after thecompletion of the scraping operation for the reduction in viscosity ofthe paste.

The present invention is not limited to the uniform induction heating ofthe whole body of the screen mask 11, and it is acceptable to make theinduction coil 28 a partially face the portion in which the separationof the solder paste 12 from the stencil is unsatisfactory among thecircuit pattern, and inductively heat only the portion.

The present invention is not limited to the noncontact inductionheating, and as shown in FIG. 9, it is acceptable to inductively heatthe induction heating section 28 in contact with the upper surface ofthe screen mask 11. In this case, the distance between the inductioncoil 28 a of the induction heating section 28 and the screen mask 11 isreduced, and therefore, the induction heating can be efficientlyeffected in a locally concentrated manner. It is to be noted that thereference numeral 28 b in FIG. 9 denotes an induction magnetic field.

There is a tendency that the inner wall surface of a through holeelongated in a direction in which the induction heating inductioncurrent flows is easily inductively heated, and the inner wall surfaceof a through hole elongated in a direction perpendicular to thedirection in which the induction current flows is hardly inductivelyheated. Therefore, as shown in FIG. 10A, it is preferable in terms ofheat generating efficiency for the through hole 11 a elongated in theX-direction to arrange an induction coil 28 c in the lengthwisedirection of this through hole 11 a and generate an induction current byflowing a current through the induction coil as indicated by the linedenoted by 28 c in FIG. 10A. Therefore, as shown in FIG. 10B, it is alsopreferable for the through hole 11 a elongated in the Y-direction toarrange an induction coil 28 c in the lengthwise direction of the hole11a and generate an induction current by flowing a current through theinduction coil as indicated by the line denoted by 28 c in FIG. 10B. Inthis FIG. 10B, if the induction coil is arranged in the direction ofarrow 28 e and an induction current flows through the induction coil,then the inner wall surface of the through hole 11 a along theY-direction does not generate much heat. In the case of a QFP (Quad FlatPackage) as shown in FIG. 10D, the lengthwise directions of the throughholes 11a of adjacent sides cross each other at an angle of 45 degrees.Therefore, it is preferable in terms of heat generating efficiency toarrange an induction coil in a V-figured shape as shown in FIG. 10C andgenerate an induction current by flowing a current through the inductioncoil as indicated by the line denoted by 28 c in FIG. 10C.

For example, it is acceptable to overlap a first induction coil forflowing a current in the X-direction as shown in FIG. 10A and a secondinduction coil for flowing a current in the Y-direction as shown in FIG.10B and then, use the coils as one induction heating section. Bysupplying electric power to either one of the first induction coil andthe second induction coil or alternately to the first induction coil andthe second induction coil in the induction heating section in which twoinduction coils are overlapped as described above, even though thethrough hole pattern is varied, an identical induction heating sectioncan flow a current through the induction coil only in the X-directionfor the through holes arranged along the X-direction as shown in FIG.10A, flow a current through the induction coil only in the Y-directionfor the through holes arranged along the Y-direction as shown in FIG.10B, or flow a current through the two induction coils alternatelyarranged in the X-direction and the Y-direction for the through holesextending in both the X-direction and Y-direction as shown in FIG. 10Cand FIG. 10D. As a result, both the through hole 11 a along theX-direction and the through hole 11a along the Y-direction can beroughly equally heated inductively for the through holes shown in FIG.10C and FIG. 10D. Even if the through hole pattern is varied as shown inFIG. 10A, FIG. 10B, FIG. 10C, and FIG. 10D, then the identical inductionheating section can flow a current through the induction coil only inthe X-direction, flow a current through the induction coil only in theY-direction, or flow a current through the induction coils alternatelyin the X-direction and the Y-direction, so that the general-purposeproperties of the induction heating section can be improved.

FIG. 12A and FIG. 12B show an embodiment of the present inventionaccording to the screen printing system, in which a filling roller 100for filling the solder paste 12 is used instead of the squeegee. In thisembodiment, the cylindrical filling roller 100 is rotated to holdprinting material, e.g., the solder paste 12 and forcibly fill thesolder paste 12 into the through holes 11a of the screen mask 11. Thecylindrical shape of the filling roller 100 may alternately be thesawtooth-shaped one having spiral grooves 100 a shown in FIG. 12A. It isto be noted that the reference numeral 101 denotes a solderpaste-scraping-use scraper in FIG. 12A.

According to an embodiment in which the present invention is applied toa dispensing system, it is acceptable to forcibly fill a print material112 such as solder paste into the through holes 11 a of the screen mask11 by means of a nozzle 111 having an extruding function of a piston 110as shown in FIG. 13A or an extruding function of compressed air as shownin FIG. 13B. In FIG. 13B, the reference numeral 112 denotes a solderpaste-scraping-use scraper provided at the tip of the nozzle 111.

The present invention can also be applied not only to screen printingbut also to other printing methods.

For example, FIG. 14 shows an embodiment in which the present inventionis applied to a direct-printing planographic transfer printing system.In this case, a print material 122 supplied in a specified pattern on aflat plate 120 is directly transferred to a specified position 115 of aboard 114 that is the object on which a print is to be formed. In thisFIG. 14, the adhesive force between the flat plate 120 and the printmaterial 122 is reduced by inductively heating a surface on which theprint material 122 is in close contact with the flat plate 120, therebyproducing an effect of facilitating the emigration of the print materialto the specified position 115 of the object 114 on which a print is tobe formed.

FIG. 15A and FIG. 15B show an embodiment in which the present inventionis applied to an offset printing system. A print material 142 issupplied from a tank 139 stored with the print material 142 to recessportions 136 a of a plate cylinder 136 by three rollers 140, therebytransferring the print material 142 inside the recess portions 136 aonto a rubber cylinder 137. The print material 142 on the rubbercylinder 137 is transferred and printed on a paper 135 that serves asthe object which is to be put between the rubber cylinder 137 and animpression cylinder 138 and on which a print is to be formed. In thisembodiment, by inductively heating the inner wall surfaces of the recessportions 136 a of the plate cylinder 136 with which the print material122 is closely put in contact, the adhesive force between the inner wallsurface of the recess portion 136 a of the plate cylinder 136 and theprint material 122 is reduced, thereby allowing the print material to beeasily transferred onto the paper 135.

Further, FIG. 16 shows an embodiment in which the present invention isapplied to a planographic intaglio transfer printing system. In thisembodiment, by increasing the temperature of an intaglio 150 itselfthrough inductively heating the intaglio 150 similar to the screenprinting system, there can be obtained the effect that the shearingforce of the print material such as solder paste 152 on the inner wallsurface of each of recess portions 150 a is reduced and thetransferability to specified positions 155 such as lands of a board 154is improved.

FIG. 17 shows an embodiment in which the present invention is applied toan intaglio transfer printing system (gravure printing system). A printmaterial such as solder paste 162 inside a tank 165 is supplied torecess portions 163 a of a plate cylinder 163 by a supply roller 166,and the print material 162 in the recess portions 163 a is transferredand printed onto a base material 160 held between the plate cylinder 163and an impression cylinder 161. In FIG. 17, the reference numeral 164denotes a doctor, and this doctor 164 scrapes off an excessive amount ofprint material 162 of the print material 162 filled into the recessportions 163 a. In this embodiment, by increasing the temperature of theplate cylinder 163 itself through inductively heating the plate cylinder163 similar to the screen printing system, there can be obtained theeffect that the shearing force of the print material 162 on the innerwall surface of each of the recess portions 163 a is reduced and thetransferability to the base material 160 is improved.

FIG. 23 shows a perspective view of an induction coil according toanother embodiment of the present invention. The induction coil is notlimited to the annular one, and the induction coil may be asquare-frame-shaped or a rectangular-frame-shaped induction coil 728.

FIG. 24 is a perspective view showing a state in which two units 728 aand 728 b of the induction coil 728 of FIG. 23 are prepared and arrangedat two corners located in diagonal positions of a board on which a QFPis to be positioned and an induction current 729 is flowed in thelengthwise direction of each through hole 11 a. The two induction coils728 a and 728 b are preferably operated simultaneously.

FIG. 25 is a perspective view showing a state in which four units 728 c,728 d, 728 e, and 728 f of the induction coil 728 of FIG. 23 areprepared and arranged at the four corners of a board on which a QFP isto be positioned and an induction current 729 is flowed in thelengthwise direction of each through hole 11 a. In this case, the fourinduction coils 728 c through 728 f are also operated simultaneously.

FIG. 26 is a perspective view showing a state in which one inductioncoil 728 g of FIG. 23 is prepared and arranged above a board portion onwhich a QFP is to be positioned and one side edge of the induction coil728 g is arranged in a form inclined at an angle of 45 degrees withrespect to the direction in which the through holes a are arranged, sothat an equal amount of induction current 729 flows through each throughhole 11 a.

The entire disclosure of Japanese Patent Application No. 8-123393 filedon May 17, 1996, including specification, claims, drawings, and summaryare incorporated herein by reference in its entirety.

Although the present invention has been fully described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various modifications ofthe disclosed invention wall apparent to those skilled in the art. Suchmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims unless they departtherefrom.

What is claimed is:
 1. A printing method comprising: positioning anobject in a printing position below a stencil; moving the objectupwardly until an upper surface of the object is in contact with a lowersurface of the stencil; supplying a printing paste to an upper surfaceof the stencil; delivering the printing paste to a printing pastereceiving portion of the stencil by a printing paste delivery device sothat the printing paste contacts a specified area of the object via theprinting paste receiving portion; heating the stencil by inductionheating after the printing paste is delivered to the printing pastereceiving portion of the stencil; and separating the printing paste,which has been received in the printing paste receiving portion of thestencil, from the stencil so as to print the printing paste on theobject, wherein the separation operation occurs after the inductionheating operation.
 2. A printing method comprising: delivering aprinting paste into an opening portion of a stencil so as to retain theprinting paste therein, wherein a viscosity of the printing paste isreduced as a temperature thereof is increased; heating a portion of thestencil, which defines the opening portion in which the printing pasteis retained, so as to reduce the viscosity of the printing paste that isin contact with the heated portion of the stencil, thereby permittingthe printing paste to be easily separated from the stencil, wherein theheating operation is performed by electromagnetic induction heating toincrease the temperature of the portion of the stencil, wherein saidheating operation by said electromagnetic induction heating is performedafter the printing paste is retained in the opening portion of thestencil; and separating the printing paste, which is retained in thestencil, from the stencil so as to print the printing paste on an objecton which a print is to be formed, wherein the opening portion of thestencil is arranged in a specified pattern for retaining the printingpaste, and the stencil and the object contact each other and then arerelatively separated following the heating operation in order to printthe printing paste onto the object.
 3. A printing method comprising:delivering a printing paste into an opening portion of a stencil so asto retain the printing paste therein, wherein a viscosity of theprinting paste is reduced as a temperature thereof is increased; heatinga portion of the stencil, which defines the opening portion in which theprinting paste is retained, so as to reduce the viscosity of theprinting paste that is in contact with the heated portion of thestencil, thereby permitting the printing paste to be easily separatedfrom the stencil, wherein the heating operation is performed byelectromagnetic induction heating to increase the temperature of theportion of the stencil, wherein the electromagnetic heating is performedby an electromagnetic induction heating unit in contact with thestencil; and separating the printing paste, which is retained in thestencil, from the stencil so as to print the printing paste on an objecton which a print is to be formed, wherein the opening portion of thestencil is arranged in a specified pattern for retaining the printingpaste, and the stencil and the object contact each other and then arerelatively separated following the heating operation in order to printthe printing paste onto the object.
 4. A printing method comprising:delivering a printing paste into an opening portion of a stencil so asto retain the printing paste therein, wherein a viscosity of theprinting paste is reduced as a temperature thereof is increased; heatinga portion of the stencil, which defines the opening portion in which theprinting paste is retained, so as to reduce the viscosity of theprinting paste that is in contact with the heated portion of thestencil, thereby permitting the printing paste to be easily separatedfrom the stencil, wherein the heating operation is performed byelectromagnetic induction heating to increase the temperature of theportion of the stencil; and separating the printing paste, which isretained in the stencil, from the stencil so as to print the printingpaste on an object on which a print is to be formed, wherein the openingportion of the stencil is arranged in a specified pattern for retainingthe printing paste, and the stencil and the object contact each otherand then are relatively separated following the heating operation inorder to print the printing paste onto the object, wherein an inductioncurrent for generating the electromagnetic induction heat flows in alengthwise direction of the opening portion of the stencil.
 5. Aprinting apparatus comprising: a stencil having an opening portion forretaining printing paste having a viscosity that can be reduced as atemperature thereof is increased; an electromagnetic induction heatingunit for heating the opening portion of said stencil to thereby reducethe viscosity of the printing paste in contact with the opening portionof said stencil, wherein, when the opening portion of said stencil isheated by said heating unit, the viscosity of the retained printingpaste in contact with the opening portion is reduced, thereby making theprinting paste easily separable from said stencil; a separation unit forseparating said stencil relatively from an object on which a print is tobe formed after said stencil comes into contact with the object tothereby print the printing paste onto the object such that the printingpaste is separated from said stencil, wherein the opening portion ofsaid stencil is arranged in a specified pattern, and saidelectromagnetic induction heating unit directly contacts said stencilduring a heating operation.
 6. A printing apparatus comprising: astencil having an elongated through hole for retaining printing paste,wherein the elongated through hole is adapted to retain printing pastehaving a viscosity that is reduced as a temperature thereof isincreased; an electromagnetic induction heating unit for heating aportion of said stencil defining the elongated through hole to therebyreduce the viscosity of the printing paste in contact with this portionof the stencil thereby making the printing paste easily separable fromsaid stencil; and a separation unit for separating said stencilrelatively from an object, on which a print is to be formed, after saidstencil comes into contact with the object to thereby print the printingpaste on the object, wherein the elongated through hole of said stencilis arranged in a specified pattern, and an induction current forgenerating the electromagnetic induction heat in said electromagneticinduction heating unit flows in a lengthwise direction of the elongatedthrough hole of said stencil.